This invention relates to a method for adding additives during the production of carbon fiber.
The manufacture of carbon fiber usually includes a step for stabilizing the organic fibers. When organic fiber is heated in a carbonizing furnace immediately to a temperature of, for example, 1,000.degree.-1,800.degree. C, the organic fiber undergoes a rapid exothermic reaction which results in breakage of fibers. In order to prevent this breakage of organic fibers, it is necessary to cause the organic fiber to gradually undergo oxidation and/or reduction. This necessity can be satisfied by stabilizing the organic fiber by heating it to a temperature of, for example, 180.degree.-300.degree. C.
Therefore in general spun organic fiber is stabilized to make it flameproof and then it is carbonized to produce carbon fiber.
In conventional methods for manufacturing carbon fiber, spun organic fiber is not immediately supplied to the stabilization step. Specifically, spun organic fiber is once either wound around a bobbin or shaken into a container until a given amount of the organic fiber is accumulated before it is made flameproof.
The reasons for separately spinning and then stabilizing the organic fiber are as follows:
(1) The spinning rate of polyacrylonitrilic organic fiber is 20-400 m/min. and that of pitch-based organic fiber is 200-1,000 m/min. Stabilizing organic fiber generally takes 0.5 -4 hours. In order to continuously feed spun organic fiber immediately to the stabilization step without breaking fibers, the rate of travel through a furnace used to make the organic fiber flameproof must equal the rate of production during the whole period of the above stabilization time and therefore the organic fiber must travel a distance of at most 240,000 m through the furnace. Use of a furnace with a length of at most 240,000 m is not possible industrially.
(2) If spun organic fiber is fed as it is to the stabilization step without being cut and then the flameproof fiber resulting from the stabilization process is continuously fed to the carbonizing process without being cut, when the travelling of the thread must be suspended in one of the above steps due to breakage of the thread or for other reason the feeding of the thread in the other steps must also be suspended. Therefore, only low manufacture efficiency can be achieved by supplying the organic fiber to the carbonizing step without cutting the fiber.
Accordingly, in conventional methods for manufacturing carbon fiber, spun organic fiber is accumulated until a given amount of the organic fiber is produced before the organic fiber is fed into a furnace in which the fiber is heated and made flameproof.
In addition, in conventional methods for manufacturing carbon fiber, aqueous solutions or emulsions of various types of additives are applied to the organic fiber at the end of spinning step.
These additives which are generally and collectively called oil, include an antistatic agent and a binder which are added for preventing any separation or fluffing of filaments which might result from e.g. static electrification of the spun organic fiber when it is used and for smoothly supplying the thread on a roller in order to secure production efficiency and stability. A parting agent for preventing the coalescence of filaments during the step for making the organic fiber flameproof by heating it in a furnace, an oxidation/reduction accelerator for accelerating a reaction for making the fiber flameproof during the stabilization step and a lubricant for preventing the fluffing of filaments during the carbonization process, and the like are occasionally added as additives at the end of spinning step.
However, despite the fact that various types of additives or oil is added to the fiber at the end of spinning step, the physical strength of carbon fiber produced by a conventional method is sometimes nonuniform.